The present invention relates to intercom systems utilizing the telephone sets and wiring on the premises of a typical telephone subscriber.
In the prior art a telephone intercom system has been described which uses the the two "extra" conductors (normally color-coded yellow and black) of the four-conductor wiring which is provided in homes, offices and other premises. These conductors are used to provide an intercom system which is a supplement to the normal central office operation and which is independent of it. The sets used in the system described in the earlier disclosures are of the "two-line" type equipped with a signaling push button and a "hold" switch which is released through a mechanical interlock when the handset is replaced.
If a customer wants to obtain this intercom service in an area where the telephone company furnishes the customer equipment, it is a simple matter for the customer to trade his standard telephone set for a two-line, plus signaling, plus hold button telephone at the telephone company as part of his acquiring intercom service. Current Federal communications policy is to have the customer own all the terminal equipment as well as the wiring on his premises. In locations using this method of operation it would be difficult for the customer to trade in his standard phone for a two-line, plus signaling, plus hold button set.
The above conditions make it desirable to provide an adapter, which can be inserted between the subscriber's standard telephone and the wiring on his premises and which contains circuitry and switching which will provide the same operational feature as the two-line, plus signaling, plus hold button phones which after modification, were used in the earlier system. One object of the present invention is to provide the circuitry and the switching which will make this possible.
If an adapter unit containing the circuits and switching to achieve the above object is used, it must contain a switching mechanism to transfer the operations from "central office" to "intercom" operation. Suppose that this switching mechanism were left in the "intercom" mode of operation after the subscriber has terminated his call and there is an incoming call. In this case the ringer in the subscriber's telephone will not operate. It is another object of this invention to provide means by which incoming ringing will operate the ringer in the subscriber's telephone even if the switching means are left in the "intercom" position. In addition to this feature a user who picks up the handset may need a reminder that he must move the switching means to the central office mode of operation before he can complete the incoming call. It is another object of this invention to provide a positive warning to the user of the telephone which indicates that he should move the switching means to the central office mode of operation.
In some cases it is helpful to the user to have indicator lights to indicate the current mode of operation of the equipment such as "central office" or "intercom" or "central office hold circuit is activated." Some currently available equipment requires either the use of a local, mains operated, power supply at the location of each subscriber's telephone or the use of six-conductor wiring through the premises. It is another object of this invention to provide such indicator lights and assorted functions over the same four-conductor wiring systems which is used for the central office and the intercom operation on the premises.
In the case of intercom operations the supply voltage for the intercom circuit may be the same, or approximately the same, as the supply voltage at the central office, i.e., normally 48 volts, supplied through, for instance, 400 ohms. When the talk current of the intercom system is connected across this supply, the voltage on the intercom circuit will drop dramatically, for instance, to some 6 to 8 volts. This reduction in voltage on the intercom circuit may make it difficult to obtain satisfactory operation of relays and indicator lights. It is another object of this invention to overcome this problem.
When the subscriber's telephone is connected to the wiring on the premises (through an adapter unit of the present invention) the application of a "hold" circuit to the central office line must be done in this adapter unit. It is desirable to have this hold circuit release automatically at the end of a conversation. It is another object of this invention to provide this automatic release.
It is desirable to have the ringer in the subscriber telephone react to the incoming ringing on the central office line and it is also desirable to have the subscriber telephone connected directly to the central office line at any time when the power on the intercom circuit at the subscriber premises fails. It is another object of this invention to provide these features.
Other objects are addressed hereinafter.